Tag Archives: dolostone

If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features. Some stand tall like towers while others appear to be irregular to rounded masses. It is common to see only the tops of these features when the lake level is low to normal.

Top of sandstone mass in Beaver Lake. Photo taken in October, 2016.

Sandstone mass along Beaver Lake. Photo taken in October, 2016.

These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties. He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape. Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst. Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand. Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell. One very large sandstone mass was seen in the White River (Arrington, 1962). It is approximately 45 feet tall! Unfortunately, it is now covered with water.

Sandstone mass in Carroll County. From Owen, 1858

Sandstone mass in the White River near Hwy 12 access to Beaver Lake. From Arrington, 1962.

So how did these features form? First, let’s define paleokarst. Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record. Karst features include sinkholes, springs, and caves. These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).

The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation. The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya). It is likely that this formation was exposed to weathering at that time. Depressions of various size, called sinkholes, developed on the exposed land surface. Later, sand filled the depressions and eventually became rock called sandstone. The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya). Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.

Why is paleokarst important, other than being interesting features to observe? Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011). We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas. But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.

Many sandstone paleokarst features were located while mapping the War Eagle quadrangle. Fifty-two sandstone masses were located around Beaver Lake. This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.

On a recent fieldtrip I realized how many great geologic features exist in the Everton Formation of northern Arkansas. Here’s a little background on the Everton Formation. The Everton Formation is named for exposures near the town of Everton in Boone County, Arkansas. All geologic formations are named for nearby geographic locations. This formation was deposited during the Middle Ordovician Period which means it formed around 470 million years ago. It crops out across northern Arkansas from Beaver Lake in Benton County to Sharp County. Depending on where you are in that portion of the state you might see sandstone, limestone, dolostone, or all three rock types.

Now let’s look at some neat features in the Everton Formation. We’ll start with stromatolites. Stromatolites are laminated structures built by blue-green algae, also called cyanobacteria, one of the simplest and earliest known life forms. Notice the mounded laminations in the photo below. These are stromatolites. The rock is a fine-grained limestone. Also notice the bumpy, weathered surface mid-photo. This is where individual stromatolites are weathering out of the rock.

The next photo shows a better look at the top of this weathered surface. Finding these fossilized accretionary structures in outcrop helps geologists determine the environment in which this rock formed – in this case, a tidal flat.

The next photo shows that modern stromatalites are still forming in similar environments today.

Next, let’s look at travertine. Travertine is a chemically-precipitated, continental limestone composed of calcite or aragonite that forms around seepages, springs, and along rivers and streams (Pentecost, 2010). Precipitation results primarily through the transfer of carbon dioxide to or from a groundwater source, which leads to supersaturation and crystal growth on surfaces. Travertine cascades and dams are present on many of the small streams that are sourced by springs issuing from the limestone and dolostone of the Everton Formation.

The first photo shows a travertine cascade over a dolostone ledge.

The second photo shows a geologist standing beside a tall travertine dam across a small creek.

Finally, have a look at these fossilized mud cracks. These formed in a similar way to modern mud cracks. These rocks were originally mud that dried out and formed polygonal cracks. These were later filled with additional mud and over time all of it lithified into dolostone. Mud cracks preserved in this manner are another clue that helps geologists determine the environment in which the sediment was deposited. Again, this would indicate a tidal flat.

Dolomite and sphalerite are two minerals present in limestone and dolostone in the lead and zinc districts of north Arkansas. Dolomite commonly occurs with the sphalerite, however it is not an ore mineral and is considered worthless. Sphalerite is the primary ore of zinc. Zinc was mined in the lower end of the Buffalo National River in the late 1800s and early 1900s. One of the largest mining communities was located at Rush, Arkansas. Zinc is used as a coating of iron or steel to protect it from corrosion. It is also used in batteries, small non-structural castings, and alloys, such as brass. This mineralization is present in the Everton Formation. It is thought that migration of warm mineral-rich fluids expelled by the pressure of the mountain building event that caused the Ouachita Mountains is responsible for the mineralization in northern Arkansas. Note the brecciated texture (angular fragments) of the rock. Open spaces, called cavities, in the rock caused the overlying rock to collapse, and break into angular pieces. Mineralized water then flowed around the broken pieces and the dolomite and sphalerite precipitated in the open spaces.

Painted Bluff gets its name from water seeping over the top portion of the bluff. This darkens the rock giving it a painted look. The rock formation that is painted is the St. Peter Sandstone. The rock formation below the painted portion of the bluff is the Everton Formation. Thin bedded limestone and dolostone layers make up the lower portion of the bluff. The rock formations are both Ordovician (485-444 million years ago) in age, however there is an unconformity between the two formations. An unconformity is a rock surface that represents a gap in the geologic record either due to a period of erosion or non-deposition. Notice the wavy line halfway down the bluff. This wavy line separates the sandstone from the limestone and is the unconformity surface. The top of the limestone was at one time the rock exposed at the earth’s surface in this area. The limestone was eroded, and then the sandstone was deposited upon it.